Escape URLs used as target for hyperlinks.

Up to now, when using an URL containing a & character, the generated XML was invalid.
Merge pull request #140 from die-antwort/escape-hyperlink-urls

Escape URLs used as target for hyperlinks.

Awesome
Added color scale initialization

This lets us pass hashes defiing cfvo and colors for color scale
conditional formatting with out the bs of hacking up the existing
defaults.
Updated DataBar conditional formatting to accept cfvo hashes in the initializer to bring the API inline with ColorScale
Patched for 1.8.7

Time to think about ending support for 1.8.7?
Added sensible defaults for color scale

You can now use Axlsx::ColorScale.three_tone or
Axlsx::ColorScale.two_tone to create pretty conditional formatting.
Updated README & version for next release
Minimized two_tone class method

This is really the same as just initializing the object, but anyone who
knows about three_tone is going to expect two_tone to be there as well.
Adds basic support for headers and footers.

Based on the discussion here: https://github.com/randym/axlsx/issues/111
Very simple implementation right now. Does not include any tests yet!

Change-Id: Ia8ec8c52f82577d05e07e21311cdb5fe5d1294be
Adds tests for HeaderFooter.

Change-Id: I77c92cb6af09a13193517adc102093c26cd0cead
Updated thin_chars and replaced cover_me with simplecov
Merge pull request #145 from freerunningtechnologies/master

Basic support for headers and footers.
Updated readme to give adammathys the respect he deserves!
Adds an example of header/footers.

Also, header and footer content is now escaped. So that the user doesn't
have to worry about that.

Change-Id: Ib35b2eca6755d9b2d9974be25887734873d6e64a
Refactored header_footer element serialization.

This adds a serializable_element_attributes helper that should be
integrated as we have time for serializing child elements that are
based on object attributes.
Patched serilalized_element_attributes to properly update value from called block
Updated readme and docs for pre 1.3.4 release
Cleaned up gemspec and removed pdf export example
Cleaned up examples directory
moved sheet view example into examples.rb